In recent years, the development of a CDMA mobile communication system has been in a prompt advance. Moreover, a commercial service based on the W-CDMA system has started, which enables the transmission/reception of large-capacity data such as moving images along with voices and still images included in the current main services. In this situation, as a group for the standardization of a third-generation mobile communication system, 3GPP/3GPP2 (3rd Generation Partnership Project/3rd Generation Partnership Project 2) has been organized, which is oriented to a system capable of realizing a higher-quality service and prepares diverse specifications according to a W-CDMA mode and others.
FIG. 38 is a schematic illustration of a configuration of a W-CDMA. The W-CDMA system shown in FIG. 38 is made up of nodes including a core network (CN) 100 including an exchange 101, radio network controllers (RNC) 102-0, 102-1, base stations (BTS: Base Transmitting Station) 103-0 to 103-5, and a mobile unit (UE: User Equipment). In this case, unless otherwise specified particularly, the node signifies a unit made to carry out functions of switching, transmission and transfer of a packet. Moreover, the nodes 101, 102-0, 102-1, 103-0 to 103-5 and 104 are physically connected through an ATM (Asynchronous Transfer Mode) transmission line.
The handover prescribed in 3GPP is for switching a radio resource (radio channel, radio carrier) such as transport channel while maintaining non-instantaneous chopping and quality in a case in which the mobile unit 104 separates from the base station 103-1 which has been in communication relation thereto before the handover and shits to a communication area of a different base station 103-2, and this switching is realized by soft handover.
FIG. 39(a) is an illustration of a state before the handover, and the mobile unit 104 shown in FIG. 39 (a) sets one branch (physical radio line or radio link) with respect to each of the base stations 103-1, 103-2 of the base stations 103-0 to 103-5 for transmission/reception of a radio frame. Moreover, the mobile unit 104 transmits the same data simultaneously to the respective base stations 103-1 and 103-2. The base stations 103-1 and 103-2 transmit the received data through an ATM transmission line to the RNC 102-0. The RNC 102-0 processes these data and transmits them through the ATM transmission line to the exchange 101 side or the other base station 103-0 to 103-5.
In addition, for the handover, the mobile unit 104 monitors the quality and others of the received data to monitor the radio situation (radio wave state) at all times for selecting and demodulating the data with a high quality. Thus, the mobile unit 104, when receiving a radio frame (radio signal) with a large electric field strength in conjunction with its movement, adds a new branch with respect to the base station 103-0 to 103-5 transmitting this radio frame.
FIG. 39(b) is an illustration of a state after the handover, and when the quality of the branch of the set base station 103-2 degrades with the movement of the mobile unit 104, the mobile unit 104 releases and deletes this branch and makes a communication with the base station 103-1.
Through this soft handover procedure in the mobile unit 104, the radio link is continuously switched without instantaneous chopping.
However, the number of channels needed for when the mobile unit 104 conducts the soft handover is at least two channels for the handover addressing node and the handover addressed node. The band needed for the transmission/reception of a radio frame becomes twice the normal band. In addition, since the band needed per channel increases with an increase in the communication data transmission rate (communication rate), in a case in which the mobile unit 104 transmits the same signal through a plurality of channels in a handover state, the bandwidth to be used per user extremely increases. This increased bandwidth enhances the interference in a radio zone, thereby causing the degradation of the communication quality and decreasing the number of users to be accommodated.
For solving the increase of the bandwidth and the decrease of the number of users through the use of a conventional technique, there is a need to improve the throughput required in each node and each transmission line and to increase the number of facilities, which leads to a considerable demerit in view of cost performance. Moreover, there are the following problems (S1) to (S5).
(S1) Since, in a high-speed data communication in the future, the bandwidth needed for signal transmission/reception will becomes extremely large and the introduction of the metered charge will advance, as the connection method, a method of setting a line at all times irrespective of the presence or absence of data will become a mainstream approach. From the viewpoint of efficiency, in the W-CDMA system, as a preferable method, only one shared channel (shared line) having a sufficiently wide band as compared with a method of a wide-band radio resource being allocated to each mobile unit 104 is prepared so that the respective mobile units 104 share and use the channel when needed. On the other hand, difficulty is experienced in simultaneously transmitting the same data to a plurality of channels at all times like the soft handover.
(S2) In this case, the mobile unit 104 is required to make the switching from the shared channel for the handover addressing node to the shared channel for the handover addressed node, and the instantaneous chopping occur in data communication while the mobile unit 104 makes the switching. Moreover, in consideration of the introduction of IP (Internet Protocol) for an transmission line and the speed-up of data communication, in comparison with a method of a large volume of data being divided into a large number of small packets and transmitted, it is preferable to employ a method in which data is transmitted to a lower node in the form of one packet and the lower node divides this data in accordance with the communication rate in a radio zone so that the divided data are transmitted to the mobile unit 104. This provides a higher transmission line service efficiency.
Therefore, in light of this efficiency and the above-mentioned instantaneous chopping, there is a possibility that, in a handover state, the data collectively received and held by a handover addressing node is left without being all transmitted.
(S3) For preventing data loss, a conventional method is designed such that an upper layer of a handover addressing node abandons the residual data of the handover addressing node itself and retransmits it according to a host communication protocol. However, an increase of this retransmission causes the degradation of the communication rate.
(S4) Likewise, the increase of the retransmission leads to an increase in unnecessary traffic, which presses the throughput (for example, the capability of protocol processing, signal transmission/reception, charge processing and others) of each node and each transmission line. That is, the cost performance of the facility deteriorates.
(S5) the real-time performance on communications degrades, which leads to the degradation of quality of service.
The present invention has been developed in consideration of these problems, and it is an object of the invention to provide a packet transferring/transmitting method and mobile communication system capable of maintaining the number of users to be accommodated while securing a wide bandwidth, sharing a line efficiently at soft handover, avoiding data residual at the employment of variable communication rate and preventing an increase in retransmission.